1. Field of the Invention
The invention relates to a process for joining blanks or structural components of steel to those of .gamma.-titanium aluminide, and it includes turbocharger components obtained by the process. The invention teaches the use of an interlayer as a connecting piece between the steel and .gamma.-titanium aluminide components. The connecting piece, made of an iron-base alloy, is first joined to the steel component by either inertia welding or friction welding or electron beam welding. The resulting connecting piece/steel body is then joined to the .gamma.-titanium aluminide component by friction welding or electron beam welding at the free interlayer surface.
2. Discussion of Background
The joining of dissimilar materials plays a critical role in advanced manufacturing technology, since different properties are required within any particular application, which properties cannot be obtained by a single material. Depending on the manufacture and operating function, different materials are used for individual elements of a machinery component or structure to achieve an optimum of economical manufacture and mechanical properties.
Turbochargers are used to improve the performance of gasoline and diesel internal combustion engines. A key component of this turbomchinery, the turbocharger rotor, consists of a steel shaft, connected at one end to a trine wheel, and at the opposite end, to compressor wheel. In a typical example, the turbine wheel, cast from a nickel-base alloy (usually Inconel 713 C), is metallurgically joined to the steel shaft by inertia welding. The compressor wheel, cast from an aluminum alloy, is mechanically affixed to the steel shaft.
Although this selection of materials for the different components of the turbocharger rotor has met the requirements of the operating conditions of current turbocharging systems, further performance improvement can be only achieved by replacing the nickel-base alloy (with density of 7.9 g/cm.sup.3) used in the turbine wheel by a lightweight material, exhibiting at least comparable high temperature strength (on a density corrected basis). Attempts made to commercialize ceramic turbine wheels (basically silicon nitride, with a density of 3.2 g/cm.sup.3) for transportation systems have met with only partial success, due to high manufacturing costs and reliability of the joint between the ceramic component and the steel shaft. However, since early 1990, cast gamma-titanium aluminides (.gamma.-TiAl, with a density of 3.8 g/cm.sup.3) have emerged as a potential substitution material for nickel-base alloys in turbocharging systems, with the resulting impetus to provide a reliable and cost effective joint between the steel shaft and the .gamma.-TiAl turbine wheel.
The steel grade selected for shaft components in thermal machinery is usually quenched and tempered steel, for example 4140 steel, because of its high tensile strength and wear resistance. When exposed to cooling from elevated temperatures, such as is the case when joining the steel shaft to a .gamma.-TiAl turbine wheel to form a turbocharger rotor, the steel experiences a martensitic transformation at the surface in contact with the .gamma.-TiAl. The nucleation of the martensite phase is accompanied by a volume expansion. A direct contact between the .gamma.-TiAl and the steel components results in a high tensile stress on the .gamma.-TiAl at the TiAI/martensite interface due to this volume expansion, causing cracking and failure of the joint.
The prior art has been e s tablished by:
Y. Nishiyama et al., "Development of Titanium Aluminide Turbocharger Rotors", iegh Temperature Aluminides and Intermetallics, edited by S. H. Whang, et al., The Minerals, Metals & Materials Society, 1990.
Brogle et al., "Friction Welding of Ga mma Titanium Aluminide to Steel Body with Nickel Alloy Connecting Piece There Between", U.S. Pat. No. 5,431,752, issued Jul. 11, 1995.
The prior art has recognized a benefit in using an interlayer as a buffer material in the friction welding of .gamma.-TiAl to steel. The Nishiyama work report s the use of a connecting piece of a nickel-based alloy Inconel 751 (Ni-0.05 C-15.5 Cr-0.95 Mo-1.2 Al-2.3 Ti-7.0 Fe) having a nickel content of over 70 percent by weight (wt%). The U. S. Pat. No. 5,431,752 pat. discloses the use of a connecting piece of a nickel-base alloy containing less than 65 wt % nickel, less than 30 wt. % iron, and 3 to 7 wt. % niobium. The '752 patent teaches that the friction welding joint of the .gamma.-TiAl component to the connecting piece can be produced at compaatively low temperatures, using a rubbing cycle lasting up to 120 seconds, preferably 60 to 80 seconds. However, this rubbing time is impracticable for high production volume.
These and other difficulties experienced with the prior art devices (chemical processes) have been obviated in a novel manner by the present invention.
Accordingly, one object of the invention is to develop a novel process, by which blanks or structural components of steel can be joined to those of .gamma.-TiAl by friction welding within a short cycle time, less than about 30 seconds, with the goal of achieving a tensile joint strength suitable for use as a turbocharger rotor in turbocharging machinery. The short cycle is commensurate with industrial practice for high volume production (in the hundreds of thousands of units per year).
Another object of this invention is to provide a turbocharger rotor, with a .gamma.-TiAl turbine wheel capable of operating at temperatures up to 700.degree. C. The room temperature strength of the steel/.gamma.-TiAl joint achieved by this novel process is suitably adequate for subsequent manufacturing operations of the turbocharger rotor, such as turning, grinding, etc.
It is still another object of this invention to provide an interlayer material which exhibits progressive compatibility between the steel and the .gamma.-TiAl blanks or components. Joining of said blanks or components is achieved with insertion of an iron-rich connecting piece, which is metallurgically compatible with each component and suitable for inertia or friction welding. The connecting piece or interlayer in the finished joint is as thin as possibly permitted by the design and operation of the turbocharger rotor, preferably less than 10 mm in thickness for turbine wheels having a blade tip diameter of 90 mm or less.
With the foregoing and other objects in view, which will appear as the description proceeds, the invention resides in the combination and arrangement of steps and the details of the composition hereinafter described and claimed, it being understood that changes in the precise embodiment of the invention herein disclosed may be made within the scope of what is claimed without departing from the spirit of the invention: